CBS

Now that marijuana has been legalized in Colorado, there are a number of new issues to iron out.

Among these: how much is too much to drive?

Driving under the influence of marijuana can cause dizziness, slow reaction time, and make drifting and swerving more likely. But scientists have not reached a consensus on how much marijuana would cause impairment. Marijuana can also stay in the system hours after noticeable side effects wear off, and unlike with alcohol, there are no widely available tests to determine if someone is driving impaired.

The National Highway Traffic Safety Administration says peak THC concentrations occur while smoking pot, and fall under 5 nanograms per milliliter of blood after about three hours. THC is the psychoactive chemical in marijuana. Washington State, which also legalized pot in the 2012 election, has set 5 nanograms as their standard for what's too high to drive. Supporters of Washington's standard say 5 nanograms is roughly equivalent to the .08 limit for alcohol, while detractors worry that frequent users would always test above the standard even if they aren't impaired.

Lawmakers in Colorado have failed on three attempts to set a THC driving standard. They say they will try again when they reconvene in 2013. Sponsors on previous legislation are also eying a 5-nanogram limit.